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Purpose of Carburetion• A carburetor’s primary purpose is to produce a

mixture of air and fuel for different engine operating conditions.

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Purpose of Carburetion

• Store

• Deliver

• Filter

• Mix air and fuel

• Meter air and fuel

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Idle Mixture screwIdle Speed Screw

Choke PlateThrottle Plate

Float

Float Bowl

Pilot Jet

Main JetPedestal

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Carburetion Air-fuel

Requirements

• Ratio of air to fuel.

• Stoichiometric ratio of 14.7:1

• Example 14.7 lbs of air to 1 lb of fuel

• Fuel ratios– Rich / lean

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Carburetor Operating Conditions

• Cold– Choke / prime

• Low speed– Idle

• Mid speed – Idle and high speed

• Acceleration– Accelerator pump

• High speed– Main discharge nozzle

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Carburetion Pressure Differences

• Vacuum• Pressure less than atmospheric pressure

• Atmospheric pressure• Force of air on a surface at 14.7:1psi

• Venturi principle• A restriction in an air passage which causes an

increase in air speed and a decrease in air pressure

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Carburetion Pressure Differences• Air horn

– Air enters at Atmospheric pressure

• Venturi– High vacuum is

created

• Engine side– Low vacuum

created on the intake stroke

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Carburetion• Atomization

– Turning a liquid into little droplets

• Vaporization– Turning liquid

droplets into a vapor or gaseous state

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Carburetion

• Types of carburetors - air direction– Side draft

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Carburetion

• Types of carburetors - air direction– Up Draft

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Carburetion

• Types of carburetors - air direction– Down draft

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Carburetor Circuits• Float circuit

– To maintain proper fuel level

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Carburetor Circuits

• Choke circuit– Provides a

rich air-fuel mixture for cold starts

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Carburetor Circuits

• Throttle – Regulates

air –fuel mixture entering the cylinders

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Carburetor Circuits

Idle circuit– Supplies air-

fuel mixture when the throttle is closed for low speed operation

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Carburetor Circuits• High Speed

circuit– Full throttle– Maximum

air – fuel flow to develop full speed and power

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Carburetor Circuits• Accelerator

circuit– Provides

additional fuel to maintain the air fuel ratio when the throttle is quickly opened.

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Carburetor Circuits• Primer circuit

– Forces addition fuel into the engine prier to cold starting

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Carburetor Circuits

• Diaphragm uses atmospheric pressure and vacuum to move fuel into a chamber of the carburetor to be used as needed

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Carburetion / Governors

• Manual throttle controls

• Governor throttle controls– Limit low speed– Limit high speed– Maintain constant speed

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Carburetor / Governor• Air vane governor

– Pneumatic

– Flywheel air pressure

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Carburetion / Governors• Centrifugal (mechanical) governor

– Rotating mechanical force

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Carburetion Air Filters

• Air cleaner purpose (3)– Filter air to remove dirt– Protect against back fire– Quiet engine operation

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Carburetion Air Filters• Foam type air cleaners (reusable)

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Carburetion Air Filters• Paper type air filters (replaceable)

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Carburetor Adjustments

• Before making any adjustments clean and inspect parts for wear !!– Rough adjustment– Fine adjustment

• Idle mixture adjustment

• High speed mixture adjustment

• Rpm adjustment

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Carburetor Adjustments• Before making any adjustments clean and

inspect parts for wear !!

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Carburetor Exploded View

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Running Conditions• Too Rich

– Air-fuel mixture with excessive fuel– HC and CO pollution– Exhaust black soot

• Too Lean– Air-fuel mixture lacking fuel– CO and NOX pollution– Engine overheating

• Just Right– Smooth engine operation– Rapid Acceleration– Low pollution / Clean Exhaust– Normal operating temperature

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Carburetion• The purpose of the fuel system is store,

deliver, mix and meter air and fuel for the engine to function over a variety of operating conditions.